1. Field of the Invention
In those sections of the country wherein total rainfall amounts to as little as 10 to 20 inches per year, in order to raise winter wheat satisfactorily, it is necessary to allow the land to lie fallow, that is, to remain unplanted during the year following the harvest of a crop of winter wheat, which harvest usually occurs in July or August. The main purpose of the practice is to conserve what moisture is provided by nature. Best results from the conservation of moisture standpoint are obtained by preventing the growth of weeds and volunteer winter wheat between the time of harvest in one year (that is, in July or August) and the time of planting the seed for the new crop of winter wheat approximately a year later, in September or October.
In those sections of the country where, because of the sparcity of rainfall, a fallow-land program must be followed in order to raise spring wheat, the wheat crop is harvested in about August. The land then is allowed to lie fallow through the suceeding fall and winter months, and through the spring, summer, fall, and winter months of the succeeding year, and then in the spring of the next year, the seed for the new crop of spring wheat is planted. Thus, there is a fallow period of about 18-22 months in the spring wheat fallow program.
The control of weeds which grow in wheat fields after the wheat crop has been harvested has long been a problem to wheat farmers. The growth of the weeds has several disadvantages. The weeds, by their presence and growth, take needed moisture and nutrients from the soil, and prevention of this loss of moisture is particularly important in winter wheat production because of the relative scarcity of rainfall in those regions where winter wheat is grown. Further, the weeds are of no use as fodder for the farm animals. When the weeds mature, the seeds which are produced at maturity are scattered by the winds, birds, and animals, so that the infestation of weeds spreads far beyond the confines of the particular field.
2. Description of the Prior Art
The usual method of controlling the weeds in fallow wheatland has been to plow the field periodically, or otherwise cultivate the field to kill the weeds. These operations kill the weeds so they do not mature and produce additional seeds, and so that the depletion of water in the soil by the weeds is prevented. The plowing and other cultivations do make the soil more friable and more receptive to rainfall, and the rainwater thereby does not run off and become lost.
However, the disadvantages of the usual method of controlling weeds in fallow wheatland are of such significance they are not to be overlooked. Control of the weeds by plowing usually requires above five plowings between the time of harvesting the wheat one year and the seeding of the new crop approximately one year later in the case of winter wheat, or about 18-22 months later in the case of spring wheat. Such plowing take time to accomplish. There is wear and tear on the machinery used by the wheat producer to accomplish the cultivations. The multiple cultivations also loosen the soil and thus permit greater wind erosion of the soil. In addition, there is the increased consumption of fuel to power the machinery used in the cultivations. There would thus be significant overall savings to the producer, together with benefit to the ecology, if the number of man and machine operations on the soil could be reduced.
While the prior art refers to many thiadiazoles and derivatives thereof, none of the prior art is believed to be an enabling disclosure of the specific use disclosed and claimed herein. Some related compounds shown in the prior art, such as, for example, 1-(5-methyl-1,3,4-thiadiazol-2-yl)-3-phenylthiourea and 1-(5-methyl-1,3,4-thiadiazol-2-yl)-3-phenylurea [J. Pharm. Soc., Japan 74, 1044-1048, (1954); C.A. 49, 11630 (1955)], were not reported to have biological activity.
An article in Farmaco Ed. Sci. 22 (6), 393-401 (1967), discloses the use of 1-(5-alkyl-1,3,4-thiadiazol-2-yl)ureas as intermediates for the production of isomeric 1,3-bis-(5-alkyl-1,3,4-thiadiazol-2-yl)ureas, which latter compounds are alleged to exhibit hypoglycemic action. These compounds are only generally related to those used in the practice of the instant invention.
In U.S. Pat. No. 3,565,901 (Feb. 23, 1971), are taught salts of certain thiadiazol-2-ylureas alleged to be useful in agricultural applications as phytotoxicants. These salts are formed by thiadiazol-2-ylureas unsubstituted on the urea nitrogen closest to the thiadiazole ring. There is no mention of possible utility of the compounds in the cultivation of winter or spring wheat, and no test data regarding safety to winter or spring wheat.
Belgian Patent No. 765,930, teaches the use of 1,3-dimethyl-1-(5-tert.-butyl-1,3,4-thiadiazol-2-yl)urea, and 1-ethyl-1-(5-tert.-butyl-1,3,4-thiadiazol-2-yl)-3-methylurea, as pre- and postemergence herbicides on plant crops such as corn, cotton, peanuts, sorghum, and sugarcane. Data in this Belgian patent show that corn, cotton, sorghum, and wheat are largely destroyed by either pre- or postemergence application of from about 1.12 to about 5.6 kg./ha. of 1,3-dimethyl-3-(5-t-butyl-1,3,4-thiadiazol-2-yl)urea, or 1-ethyl-1-(5-t-butyl-1,3,4-thiadiazol-2-yl)-3-methylurea. The Belgian patent makes no suggestion that modification of the therein disclosed compounds to the structures disclosed in the instant application would yield compounds useful in the control of weeds in fallow wheatland.
In addition, Belgian Patent Specification No. 744,812, teaches substituted 1,3,4-thiadiazoles, and alleges their utility as preemergence and/or postemergence herbicides, and as defoliants and dessicants of plants. No teaching of utility as selective herbicides appear to be present.
British Patent Specification No. 1,195,672, published June 17, 1970, teaches thiadiazoles alleged to possess utility as pre- and/or postemergence herbicides, together with herbicidal compositions containing the thiadiazoles as active ingredient, and a method of controlling plant growth. No suggestion that any of the compounds would be suitably selective for use as herbicides in the cultivation of winter wheat is present and winter wheat is not mentioned in the patent.
Yet another British Patent Specification No. 1,230,432, published May 5, 1971, is concerned with N-substituted 5-amino-1,3,4-thiadiazoles, the processes for their production, and their use as herbicides. A number of plants, including cotton, carrots, coffee, beans, beets, sugarcane, potatoes, bluegrass, barley, and wheat, as well as mustard, common chickweed, fescue, foxtail, and the like, are listed as susceptible to control by the N-substituted thiadiazoles. Thus, the control of desirable crop plants is made synonymous with the control of weed plants.
Another reference, British Patent Specification No. 1,254,468, published Nov. 24, 1971, is directed to 5-substituted 1,3,4-thiadiazolylureas, to processes for the production thereof, and to herbicidal compositions containing the named compounds. Utility for the control of weeds and wild grasses is alleged. No teaching is present of possible selectivity of herbicidal action of the compounds, nor utility in the cultivation of winter or spring wheat.
British Patent Specification No. 1,266,172, published Mar. 8, 1972, is directed to substituted thiadiazole compounds and alleges their utility as herbicides, fungicides, acaricides, or insecticides. There is no allegation in the British patent that the compounds listed therein would be useful as herbicides in the cultivation of winter or spring wheat.
Kubo et al., J. Agr. Food Chem. 18 (1) 60-65 (1970), teach the results of the study of the preemergence and postemergence herbicidal activity of a number of 1-substituted-3-(5-substituted-1,3,4-thiadiazol-2-yl)ureas. The compound, 1-methyl-3-(5-t-butyl-1,3,4-thiadiazol-2-yl)urea, is reported as a strikingly phytotoxic compound. These authors report that this compound completely killed wheat when applied at the rate of 5 kg./ha. postemergence, but that it caused only slight damage to the wheat when applied at the same rate preemergence the day the seed was planted. However, preemergence tests in our laboratories using this compound at rates of 1.12, 2.24, and 4.48 kg./ha. killed the wheat.
Also in the prior art is British Patent Specification No. 1,297,147, published Nov. 22, 1972, which teaches 1,3,4-thiadiazolyl-(5)-ureas and their use as pre- or postemergence herbicides. The compounds are alleged to be particularly well suited for selective weed control in cereals, cotton, and carrots. In general, the application rate is taught as being from 1 to 50 kg./ha., preferably from 2 to 20 kg./ha. There is no teaching or disclosure suggesting use on fallow wheatland in the cultivation of winter or spring wheat. In general, with few exceptions, the compounds tested killed wheat plants when applied either pre- or postemergent, according to the data disclosed in the patent.
Another prior art reference is U.S. Pat. No. 3,726,892 (Apr. 10, 1973), directed to thiadiazol-2-ylureas containing a sulfonamido group in the 5-position. The compounds are alleged to be active as agricultural pesticides, particularly herbicides. There is no teaching that the compounds taught therein would be usable in the cultivation of wheat in a fallow wheatland program.
Another prior art reference is Eue et al., entitled "A New Total Herbicide of the Thiadiazole Base," Proc. 7th C.O.L.U.M.A. Conference, Versailles, France, Dec. 13-14th, 1973, Vol. I, pp. 14-22. This reference teaches that 3-(5-ethylsulfonyl-1,3,4-thiadiazol-2-yl)-1,3-dimethylurea is an excellent total herbicide, having very long activity and extremely broad activity, when applied either pre- or postemergence. The compound is only recommended for use on roads, railroad tracks, and industrial sites. The authors teach that use of the compound for selective weed control in crops is not possible.
Yet another prior art reference is Muller et al., entitled "1,3-Dimethyl-3-(5-trifluoromethyl-1,3,4-thiadiazolyl-(2))-urea, A New Substance for Total Weed Control," Proc. 7th C.O.L.U.M.A. Conference, Versailles, France, Dec. 13-14th, 1973, Vol. I, pp. 22-32. The reference teaches the compound is reserved for purely "industrial" weed control where long lasting activity and total weed control is desired. The compound shows a long and nonspecific efficacy against a wide range of mono- and dicotyledons, including shrubs and bushes, according to the reference. There is no teaching that modification of the disclosed compound to the structures of the instant application would make such compounds useful in fallow wheatland weed control.
The search continues for a satisfactory method of controlling the growth of unwanted vegetation in fallow winter or spring wheatland, which method reduces the need to cultivate the land numerous times.